Directional antenna array



y 1941- A. ALFORD DIRECTIONAL ANTENNA ARRAY 4 Sheets-Sheet 1 OriginalFiled April 30, 1937 F IGJ.

.FIG.2.

ATTORNEY July 8, 1941. LFO D 2,248,800

DIRECTIONAL ANTENNA ARRAY Original Filed April 30, 1937 FIGA.

4 Sheets-Sheet 2 INVENTOR 444095 4L1 0RD ATTORNEY 2 July 8, 1941. ALFQRD2,248,800

DIRECTIONAL ANTENNA ARRAY Original Filed April 50, 1937 FIG. 9.

4 Sheets-Sheet 3 BY 3 a ATTORNEY keen/Aer HIV/1RD A. ALFORD DIRECTIONALANTENNA ARRAY M K M .8

I I I July 8, 1941.

NN K

, II ATTORNEY so 22) M/DFGREES 40 20 44 64: orfz r Patented July 8,194-1 U l'lE.

STATES PATENT OFFICE Mackay Radio and '1 York, N. Y., a corporatOriginal application April 140,039, now Patent No.

1940. Divided and this application Septem- Serial No. 297,063

ber 29, 1939,

2 Claims.

This invention is a division of my copending application 140,039 filedApril 30, 1937, and relates to directional antenna arrays and pertainsmore particularly to antenna arrays utilizing a parasitic reflector andadapted for the transmission of a plurality of different frequencies atthe same time.

It is an object of my invention to provide an antenna array which issimple in construction and still is adapted for the transmission of aplurality of different frequencies either simultaneously or separately.

Heretofore it has been customary to use the so-called dipole orhalf-wave antenna for the transmission of only a single frequency. Infact the very name of the antenna itself, expresses the thought that itis to be used for a single frequency only. But such antennae are, intruth, not extremely sharp in their transmitting qualities being, on theother hand, relatively aperiodic and adapted for the transmission of afairly wide band of frequencies. The only difference between an antennawhen it is operated as a true halfwave or dipole antenna and when it isenergized with a frequency different from a half-wave value lies in theimpedance which the antenna presents to the transmission line. In thecase where the radiated frequency is such that half-wave operationresults, the impedance presented by the antenna to the transmission asif a different frequency is transmitted the impedance will contain acapacitative or inductive component. But if proper provision is made forthe matching of the antenna to the transmission line over the range offrequencies which it is desired to transmit, it will be found that theso-called half-wave type antenna, consisting of two equal halves fed atthe center by means of a two-wir transmission line, may be operated inan efficient manner over at least a two to one range of frequencies.

Just as directional arrays may be constructed of a number of half-waveelements, so can they also be constructed of similar elements of otherthan a half-wave length. For the sake of simplicity such antennae aslast mentioned will be called pseudo half-wave antennae or pseudohalf-wave elements. A number of pseudo halfwave elements may be arrangedin line and be fed in phase so as to produce a highly concentrated but abi-directional radiation pattern. Unlike the ordinarily accepted halfwave devices of the prior art, such an array is adapted for thetransmission of two or three or more preselected frequencies. Generallyspeaking, an array of this line is resistive whereelegraph Company, New

ion of Delaware 30, 1937, Serial No. 2,195,880, dated April kind wouldhave a maximum gain at that frequency at which the length of theindividual pseudo half-wave element would be in the neighborhood of 1.2wavelengths and would be somewhat less at all other frequencies. Thedecrease in gain with a change in frequency starting from the frequencyof maximum gain is rather slight.

The main advantage of this multi-frequency array is relatively high gainper unit space, while a disadvantage is the fact that the array isbidirectional. This latter disadvantage is particularly serious when adirectional array is to be used at the higher frequencies and forcommunication over relatively long distances. The bi-directionalcharacter of the radiation frequently results in the so-called echodifiiculties.

In order to reduce the back radiation of a multi-frequency array ofpseudo half-wave elements it is necessary to provide a suitablemultifrequency reflector. While it is possible to install anothermulti-frequency array directly in back or directly in front of the firstarray and identical to it, and feed it in such a way that the radiationin the backward direction will be completely cancelled, I prefer, inaccordance with my invention, to utilize a parasitic reflector. By theuse of a parasitic reflector the difficulty of supplying a feedingsystem for the second or refleeting array, is avoided.

The usual type of parasitic reflector, consisting simply of a length ofwire slightly longer than an electrical half-wavelength, is not suitablefor use in a multi-frequency array since this type of reflector willfunction at one frequency only.

In accordance with my invention I provide a pseudo half-wave antennawhich is associated with a novel type of reflector, the latter beingadapted to operate at a plurality of difierent frequencies. Thisreflector instead of being a single length of wire slightly longer thanan electrical half-wavelength, is similar to the pseudo half -waveantenna, consisting of two equal halves connected to a sharp balancingor phase adjusting section of transmission line at the center, the twohalves having an over-all length which is about the same as that of thepseudo half-wave antenna itself. The balancing or phase adjust ingtransmission line is provided with auxiliary wires which may be eitherof the short circuit or open circuit type to insure the correct phaserelationship of the reflected waves at the several frequencies which itis desired to transmit.

The above mentioned and further objects and advantages of my inventionand the manner of attaining them, will be more fully explained in thefollowing description taken in conjunction with the accompanyingdrawings; in which Fig. 1 shows a pseudo half-wave antenna; Fig. 2 is afield strength curve for such an antenna; Fig. 3 is a diagram used inexplaining the operation of an antenna according to my invention; Figs.4, 5 and 6 are forms of pseudo half-wave dipoles; Figs. 7 and 8 areend-on arrays of such dipoles; Figs. 9, 10 and 11 are reflectorstructures according to my invention, suitable for operation at severalfrequencies and Figs. 12 and 13 are curves showing the relation ofimpedanceto the square of the field strength for the reflectors.

Fig. 1 shows a pseudo half-waveantenna i fed with a source of unitpowenZ. i.The antenna impedance is assumed to beimatchedata givenfrequency by the matching device 3 to the surge impedance of thetransmission line and the impedance of the source of unit power 2 isassumed to be the same at all frequencies. When the arrangement in Fig.1 is operated at a number ofdifferent frequencies itwill be assumedunless otherwise stated, that the matching device 3 is such that theantenna impedance remains matched to the transmission line at eachfrequency. Under the assumed conditions the pseudo half-wave willradiate, if the resistance of the wires isneglected, a unit of power ateach frequency.

Then,- if an antenna of length L were in space so that theeifects ofground reflection could be neglectedthe field'producedby the antenna ata distant point located in the planebisecting the antenna at rightangles would vary with frequency in the manner shown in Fig. 2. In thisoverall length of figure the abscissa LA is the the antenna in terms ofwavelength and is thus proportional to frequency. The ordinate isproportional to the field strength at the distant point.

Fig.2 clearly shows that the pseudo half-wave antenna is per se capableof producing comparable fieldstrengthat the distant point over a widerange of frequencies.

When the ohmic resistance of the conductors is considered this range offrequencies over which the antenna can function successfully is somewhatreduced. Indeed when L/% is less than .5 the radiation resistance on acurrent loopbasis falls rather rapidly as LA is decreased, with theresultthat the currents produced by the unit power inthe section oftransmission line between the antenna and the matching device 3 isincreased. The current in the matching device.3 also increases. Thus fora given power delivered to the antenna the PR losses increase as L/xdecreases so that for very small values of L/X the field produced at thedistant point falls off and approaches zero. With the usual type ofconstruction this decrease in radiation efiiciency does not take placeimmediately upon a decrease of L/A below .5, but takes place graduallyand may be considered to be relatively small when L/A is more than .25.

.Thus in actual practice a pseudo half-wave antenna may be considered tobe an efficient radiator fromL/A of say .3 up to L/A=1.45 or almosta.5:l frequency range. Over a two to one frequency range from .7 to 1.4it is always more efficient than a. half-wave.

When the antenna has been installed at a certain height, above.groundthe effectiverange of frequencies which may be used forcommunicating with a givenpoint may be still further re- .duced undercertain circumstances because the producedfor smaller values of correctvresults may usually sidering. each pseudo half-wave antenna as twoseparate half-wave antennae spaced a distance maximum of radiation maytake place at the desired angle to the horizon at one frequency and atsome other undesired and ineffective angle at a second frequency. Whenthe two frequencies are used for communication with two different pointswhich are approximately on the same great circle this may or may not betrue. This situation is common to all kinds of antennae operated at morethan one frequency and is not a peculiarity of the pseudo half-waveantennae or pseudo half-wave arrays, and it need not be discussed herein detail.

The pseudo half-wave antennae which have lengths between .8)\ and IAOAare particularly useful as elements in arrays because they result ingreater signal strength at the distant point per feeder.

Inorder that the action of these pseudo halfwaveantennae in arrays maybe more clearly understood the following elementary explanation oftheaction cf-such anantcnna with L/ l.0 is

' offered.

It .is well known that the field produced at the distant point byanarray consisting of two half-wave antennae which are fed individuallyand placed end-on as shown in Fig. v3 depends on the distance S betweenthe half-wave elements. :For small values of S the mutual impedancebetween. the elements tends to reduce the currents produced in thehalf-wave elements by a unit of power and thus decreases the field atthe distant point. For small values of S the field at a distant pointincreases as S is increased.

:A pseudo half-wave such as shown in Fig. 4 hasa current distributionwhich is very similar to that of two separate half-wave antennae shownin Fig. 3. The only difference between the .two current distributions isthe additional current in opposite phase indicated by the shaded area inFig. v4. When. distance S is relatively small the current in the shadedareas is also small and the field produced by this current at thedistant pointis likewise quite small. This field, howeven'tends tooppose the field produced bythe main radiating portion of the structure.Thus asLzis increased beyond 1.0x, at first the effect isrto decreasethe mutual impedance be tween the two halves of the antenna and thus toincreasethe field produced at the distant point-bya transmitter of .unitpower. When L isstill furtherincreased the radiation from the shadedareain Fi .4 .becomes appreciable and begins to cancel. out themainradiation thereby decreasing :thefield'H at the distant point. is thisphenomenon which causes the decrease of thefieldH when L l.30 Thiseffect may be-made to take place for values of.L l.3 by cancelling outsome of the radiation from the shaded portion in:Fig. thy providing anauxiliary radiator i-4 energized 180 out of phase with the mainradiator, as shown inFig. 5.

On the other hand the peak radiation may be L by loading the ends oftheantenna with some form of capacity .5-5, for example as shown intFig.6.

When a numberof .psuedo half-wave antennae are operated together in anarray it becomes necessary to insurethat they cooperate with each otherin such a way as to produce maximum field at the distant point. Theexact calculation of rmutual interaction between pseudo half-waveelements is rather complicated but approximately be obtained by con-S=L- This assumption becomes more nearly correct when an auxiliaryradiator as shown in Fig. is employed. When such an auxiliary radiatoris not used then the error in the calculation is due to the fieldproduced by the auxiliary radiator. As this field is usually not verylarge it may be neglected in the calculation of interaction between theelements.

Thus it is found that when two pseudo halfwave antennae are connected inbroadside and parallel to each other they should be about .6 to .75)from each other for best results. Separations between ends of .2A to.67\ are found to be satisfactory when two pseudo half-wave antennae arein broadside and in line with each other.

From the above discussion it is also clear that a broadside antenna suchas shown in Fig. '7, consisting of a number of sections 6, I, 8, 9, eachapproximately 154% long, which radiates maximum energy in the plane atright angles to the antenna produces greatest radiation when the phasechangers iii, H] are so adjusted that the phase delay is about and not xas is generally assumed. This type of an antenna becomes less and lessaperiodic per se as the number of half-wave radiating portions isincreased. Moreover, when a large number of radiating portions areemployed the phase changers located further away from the ends alsobegin to radiate. For this reason it is probably best to use coilsinstead of loops as phase changers in the elements next to the feeder.

It has already been explained that pseudo halfwave antennae per se areefficient radiators over a fairly wide band of frequencies. cordancewith the teachings of my copending applications, Serial Nos. 12,451filed March 22, 1935, 18,995 filed April 3, 1935, and 118,886 filedJanuary 2, 1937, the impedances of these antennae may be matched to theimpedance of the transmission line at a plurality of frequencies undersubstantially the conditions herein above assumed. Moreover, by usingthe procedure explained in my copending application, Serial No. 118,886,it is possible to operate a pseudo halfwave antenna or a number of'themon several frequencies simultaneously.

In order that echo phenomena on the longer radio circuits may be reducedit is usually necessary to provide an antenna which is substantiallyunidirectional. To achieve this end an array of pseudo half-waves may beprovided with a reflector. This reflector may be either of the fed or ofthe parasitic type. When the reflector is fed it is usually made similarto the radiator in every respect and is placed at some fraction of theoperating wavelength behind or ahead of the radiator and is fed in suchphase that the back radiation is cancelled.

When an array of pseudo half-waves is operated at more than onefrequency reflectors of the parasitic type are usually easier to tune,require less material and as a rule provide sufficiently high front toback ratio for practical purposes. For these reasons this type ofreflector will be described in some detail.

The parasitic reflector which is well known consists simply of a lengthof wire placed in back of the radiator anywhere from a 2x to .SA andadjusted to such length that the back radiation from it in the backdirection is about 180 out of phase with the radiation from the antenna.Parasitic reflectors of this type are suitable atone frequency only.

Now, in acmagnitude and of such A type of parasitic reflector which issuitable for use at several frequencies is shown in Figs, 9 and 10. Thisreflector consists of a pseudo halfwave antenna H, a length oftransmission line I2 and a number of auxiliary sections l3, 14 oftransmission line shunted across the main transmission line. When such areflector is located a fraction of a wavelength behind the radiator thenbecause of the mutual impedance between them there is produced in thereflector a certain voltage E, referred to some convenient point such asa current loop. This electromotive force in turn produces a currentwhich depends on the total impedance of the reflector. This totalimpedance consists of two parts: the impedance of reflector itself Z1and the impedance of the transmission line Z2. At a given frequency theimpedance Z1 remains fixed as long as the pseudo half-wave antennaitself is unchanged. The impedance of the transmission line Z2, however,may be varied at will, for example, by varying the position of the shortcircuiting member [5 in Fig. 10. As Z2 is varied the current in thereflec tor is also varied in magnitude as well as in phase, the currentfollowing the formula Thus by adjusting Z2 it is possible to produce acurrent which is of approximately correct phase and amplitude in thereflector provided that voltage E has suiiicient magnitude andapproximately correct phase. This condition exists when the reflector islocated between .1 and .3)\ from the antenna. However, as will bementioned later certain spacings are preferable to others, moreover, insome cases, depending on the length of the antenna and of the reflectora spacing of somewhat less than .1) may be utilized.

Assuming the induced voltage E is of sufficient phase that the range ofphase adjustment provided by Z2 is sufficient for producing the currentof the correct phase, then it follows that by moving the short I5 inFig. 11 into various positions any value of Z2 between -7'. and +a'. maybe obtained. Thus for a given frequency h the reflector is in adjustmentwhen the short i5 is in some position A. At another frequency f2, thereflector is in proper adjustment when the short is in some otherposition B. If it is desired to have the reflector in adjustment at bothfrequencies at the same time, the simple short would no longer do as itwould have to be in both positions simultaneously. In order that thisresult may be achieved in effect the system shown in Fig. 9 may beemployed. In this figure the short in position A has been replacedby abuilding out section 13 and the length of the open-ended line it is madeAA at frequency f1.

The operation of this structure is as follows: at frequency ii thequarter wave line it produces a nearly zero impedance at A and thus actssubstantially as though there were a physical short at A. This action ofthe quarter wave line is in no way disturbed by the presence of thebuilding outsection it since the total impedance obtained by parallelinga finite impedance across a zero impedance is, of course, always zero.For this reason the length of section [3 may be varied at will withoutin any way disturbing the operation of the tuning of the reflector atfrequency f1. On the other hand, the impedance Z2 at the secondfrequency it may be varied by changing the length-of section l2. Infact, the length of this section may be so adjusted as to produce thesame value of Z2 which would result if there were a physical short inposition A. Assuming that the distance between position A and position Bin degrees at frequency f2 is and assuming further that n is thewavelength at frequency f1 and that A2 is the wavelength of frequency f2and that the length of section l3 in degrees of frequency f2 is 0.

Then in order that the impedance at B be equal to zero at frequency f2it is necessary that the impedance at A at the same frequency be equalto cot (90).

The impedance which is actually obtained at this point is Z which isgiven by the following equation:

1 1 1 (1) 2 3 tan j cot (90 j cot 0+j tan (90%) when section l3 has beenproperly adjusted Z=7' cot (90) so that This equation enables thecalculation of 0 for any given f1 and Thus 0 may be found from 0 S-m-AZwhere 0 is in degrees and S in same units as )2.

When the reflector is to be tuned to three frequencies there will bethree positions of the short which have to be considered. Suppose thatthese positions for frequencies 71, f2, is, are respectively A, B andC-Figs. 9 and 11. This tuning may be accomplished as follows: The firststep is to determine the points corresponding to A, B and C, at whichshorts would properly tune the reflector at the respective frequenciesf1, f2 and fa. This is done by moving up or down the transmission lineof the reflector a short circuiting member, while energizing with thedesired frequency the antenna with which the reflector is associated,and at the same time measuring the backward radiation to determine theminimum value. This value shows the proper location for the shortcircuit. Or conversely the maximum forward radiation may be used todetermine this location, if the forward radiation is the primeconsideration. The three points A, B and C are then marked on thetransmission line.

The next step is to install a quarter wave section at frequency 11 at Aand to calculate the length S of a shorted building out section whichconnected at point A would tune the reflector to In as has already beenexplained above. Then instead of installing this shorted building outsection of length S an open section of length S +1 is installed. Afterthis is done the reflector will be tuned to two frequencies f1 and f2.Now in order to tune the reflector to the third frequency a section oflength D is connected across the section of length A2 8 +11 at the pointwhich is A2 I from the open end of that section. This new section oflength D will not disturb the operation of the reflector at frequenciesf1 and f2 but will permit the tuning at the third frequency.

The length D of the new section may be calculated by assuming for themoment, that only frequencies f1 and is are concerned. Then the length Sof the building out section which would tune the reflector to frequencyis may be calculated from Equation 2.

When this phantom length S is known, the desired length D may becalculated at once. In fact the impedance presented by the shortedsection of length S at frequency is is equal to 21r J tan 5 In orderthat the complex section consisting y 4 D and 0 present the sameimpedance at frequency is as a simple shorted section of length S, thefollowing equation must be satisfied j tan and hence from which D may becalculated. When the length D calculated from Equations 2 and 3 comesout longer than a quarter wavelength it may be shortened by a quarterwavelength and left open, instead of shorted at its far end.

So far it has been shown how the reflector may be tuned to a number ofdifferent frequencies so that it would produce a maximum amount offorward radiation or a minimum amount of backward radiation for a givenposition of the reflector with respect to the antenna. As alreadyexplained above the voltage which induces a current in the reflectordepends in magnitude and phase on the spacing between the reflector andthe radiator. While for any given length of the radiator and thereflector there exists the best value of distance between them whichresults in maximum radiation forward and still another distance betweenthem which results in a maximum amount of back radiation, when thecomplete aerial consisting of a radiator and a reflector is to beoperated at a numbe of frequencies a compromise must be had.

From experiments with such aerials I have found that in order to obtaina structure which is efficient over a range of frequencies of 2 to 1 itis best to choose such spacing between the reflector and radiator thatit is equal to approximately .2A 0 .22A at the highest frequency atwhich the aerial is to be operated. Under such conditions it is foundthat good reflector action is obtained at twice the said lowestfrequency.

The aerial consisting of a radiator and a reflector is not necessarilymost efiicient when the length of the radiator alone would be mostemcient, for example it is found that when the spacing between thereflector and radiator is .2 the aerial is more efflcient when both theradiator and the reflector are 1A long rather than 128A long. On theother hand the latter condition results in greater front to back ratio.

Figs. 12 and 13 in which the abscissa represent the phase angle of Z1+Z2which is the total impedance of the reflector while the ordinates showthe field squared which is produced in the forward direction and also inthe backward direction. It is seen from Fig. 12 that when the phaseangle Z1+Z2=0 the radiation in a forward direction is a maximum and whenthe phase angle of Z1+Zz=40 the radiation in a backward direction is aminimum. The maximum front to back ratio is obtained with the phaseangle of Z1+Z2 between these two values.

The maximum radiation forward obtained with the aerial corresponding toFig. 13 is also obtained when the phase angle of Z1+Z2=0 but the minimumback radiation occurs when this angle is about 23. Thus the minimum backcondition is more nearly equal to the maximum front condition and a muchhigher front to back ratio is obtainable with this aerial. The maximumforward radiation obtainable is somewhat less than that obtainable withthe antenna corresponding to Fig, 12.

It may be noted tion of maximum nearly twice the power that in bothcases the condiforward radiation results in being radiated in a forwarddirection clearly showing that with parasitic reflectors much longerthan one-half wave length when excited by radiators of the same lengthit is possible to obtain approximately the same or in fact a betterreflector action than is obtainable with a parasitic reflector a halfwave length long energized by a half Wave antenna well known in the art.It may be seen that the antenna of Fig. 12 when operated at half thefrequency becomes a half wave with a reflector located .1 from theradiator and which as is well known can be adjusted to deliver aconsiderable forward radiation with a high front to back ratio. I havefound from experiments that also with radiators and reflectors oflengths intermediate between 1.3x and .5) it is possible to secureforward gain with a ratio when the reflector radiator spacing is 22x atthe highest frequency.

high front to back about At times it may be desired to retune anexisting aerial consisting of a pseudo half-wave antenna with a pseudohalf-wave reflector of the type herein disclosed at some frequency atwhich the spacing is greater than 22k. Under these conditions it will befound that the reflector can still be tuned so as to increase theforward radiation at the expense of side and back radiation. This actionwill not, however, be quite as pronounced because of the comparativelylow current induced in the reflector.

A number of aerials, each consisting of a pseudo half-wave radiator anda pseudo halfwave reflector may be arranged in broadside or in line andoperated as an array. When arranged in broadside it is best to keep thespacing between the ends of the adjacent antennae 2x or greater at thelowest frequency so as to avoid the interaction between the variousradiators and reflectors which makes the tuning of the variousreflectors dependent on each other and for that reason a somewhatlaborious procedure.

While I have described particular embodiments of my invention for thepurposes of illustration, it will be understood that variousmodifications and adaptations thereof may be made within the spirit ofthe invention as set forth in the appended claims.

For example while in the above description antenna arrangementsembodying my invention have been described in connection withtransmission the same arrangements may be used for reception of signals.Furthermore while the tuning of the reflector to three differentfrequencies has been described it should be understood that tuning tomore than three frequencies may be carried out by a repetition of theprocedure already described.

What I claim is:

1. An antenna array comprising a radiator and a reflector, and means fortuning said reflector simultaneously to a plurality of differentfrequencies, the spacing between said radiator and reflector being notover .25 at the highest frequency and greater than .08)\ at the lowestfrequency.

2. An antenna array comprising a radiator and a reflector, each havingtwo radiating elements of approximately equal lengths, said reflectorhaving a transmission line connected thereto intermediate said lengths,said line terminating in a plurality of auxiliary lines of differentlengths, the number of said auxiliary lines being at least equal to thenumber of frequencies at which it is desired to use the array.

ANDREW ALFORD.

